In machines, such as earth moving equipment having hydraulic steering systems it is desirable that a secondary supply of fluid be provided to permit more complete control of the machine should an interruption occur in the primary supply of hydraulic fluid. The primary supply is normally provided by an engine driven pump of any conventional design.
In currently known machines it is common practice to provide a ground driven pump which supplies hydraulic fluid to the steering system due to an interruption in supply from the primary system due to failure of the engine or the pump itself. Other known systems utilize a separate pump driven by an electric motor to provide a supply of fluid when the primary supply is interrupted. These systems are adequate for supply only to the steering systems and/or the standard brake systems.
In certain types of machines, such as off-highway trucks, it is desirable to provide dynamic braking or retarding to assist in controlling the speed of the machine when operating in downhill conditions. In machines having standard retarding, such as by oil cooled disc brakes, the provision of supplemental dynamic braking can reduce the heat rejection load on the brakes. This allows a reduction in the cooling oil flow required through the oil cooled disc brakes and may also allow a reduction in the size of the oil cooled brakes.
U.S. Pat. Nos. 3,613,818 issued Oct. 19, 1971, 3,851,721 issued Dec. 3, 1974 and 3,935,918 issued Feb. 3, 1976, all commonly assigned to the assignee of the subject invention, teach the provision of a ground driven pump to provide a secondary supply to the steering system or steering and brake systems in the event of failure of the primary fluid supply. None of these systems provide supplemental dynamic braking in combination with the secondary oil supply to the steering system.
Thus, it is desirable to provide a hydraulic system including a ground driven pump to provide a secondary supply of oil which is effective to maintain the operativeness of the steering system and also provide dynamic braking or retarding. Such a system will allow the operator to selectively maintain the desired direction of the machine as well as more complete control of the machine speed.